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Ground-water model of spray irrigation in New Garden Township, Chester County Pennsylvania
Heebner, David F.
Heebner, David F.
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Thesis/Dissertation
Date
1999
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Earth and Environmental Science
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Heebner-Thesis-1999.pdf
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https://doi.org/10.34944/jjt3-0a91
Abstract
I used numerical modeling to simulate the flow of ground water at a spray irrigation site. The USGS code MODFLOW was used to create two and three-dimensional steady-state simulations, and three-dimensional transient simulations. The purpose of the ground-water model was to investigate the complexity in the parameters used in the model. The model parameters included horizontal hydraulic conductivity, vertical hydraulic conductivity (represented by the MODFLOW variable VCONT), irrigation rate, precipitation, horizontal anisotropy (the ratio of and Ky), and bedrock topography. Site-specific data was not readily available, so there was uncertainty in parameter values. Sensitivity analysis was conducted to evaluate the complexity needed in the parameters to accurately model the hydrologic system at the spray irrigation site. Two methods of sensitivity analysis were used: systematic variation of two parameters (while holding others fixed), and Latin hypercube sampling. Both steady-state and transient simulations were performed. Transient modeling showed the same trends as steady-state modeling with the systematic variation of hydraulic conductivity and recharge.
The sensitivity analysis showed that the hydraulic conductivity was an important parameter in predicting the effects of spray irrigation, where the model was most sensitive to values of hydraulic conductivity less than 25 m/day. This sensitivity to hydraulic conductivity has implications for data collection in that if the hydraulic conductivity of the top layer is 25 m/day or less, then more detail in hydraulic conductivity distribution and mean values are necessary to understand the effects of spray irrigation on the hydrologic system of the area. If the hydraulic conductivity values are found to be greater than 25 m/day, then detail is not as necessary. The three-dimensional modeling was beneficial in understanding the role that the soil plays in stream discharge from spray irrigation. Both the recharge to the regional ground-water system and the stream discharge are important in evaluating spray irrigation. Performing sensitivity analysis before site investigation can aid in the collection of field data, and to help make predictions about the effectiveness of spray irrigation at a particular site.
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